The present invention relates to the prophylaxis and treatment of degenerative diseases of the nervous system by the administration of an active substance which causes an increased concentration of melanin, melanin precursors, melanin derivatives, melanin variants and melanin analogs in the effected nervous system tissue. Such active substances include melanin, melanin precursors, melanin derivatives, melanin analogs, melanin variants, the enzyme tyrosinase, which catalyzes the reaction wherein naturally occurring melanin precursors are converted to melanin, tyrosinase gene, melanin-concentrating hormone and combinations thereof. Examples of such diseases include Parkinson's disease, Alzheimer's disease, retinitis pigmentosa and dementia. The present invention also relates to the treatment by the administration of melanin, melanin precursors or a melanin derivative of diseases of tissues which share a common embryological basis as tissues of the nervous system. The invention further relates to a method of preventing toxin-induced neurodegenerative diseases, toxin-induced diseases, or the adverse effects of toxins, and to a method for aiding the recovery of injured neurons, by the administration of the active substance which causes an increased concentration of melanin in the effected tissue.